1. Field of the Invention
This invention relates to novel methods of preparing an aromatics processing catalyst, the catalyst itself, and the use of said catalyst in the processing of aromatics, particularly in xylene isomerization.
2. Description of the Prior Art
Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining" volume 4 page 433 (Interscience Publishers, New York, 1961). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are as follows:
__________________________________________________________________________ Density Freezing Boiling lbs./U.S. Point .degree.F. Point .degree.F. Gal. __________________________________________________________________________ Ethylbenzene -139.0 (-95.degree. C.) 277.1 (136.2.degree. C.) 7.26 (871.2 g/liter) P--xylene 55.9 ( 13.3.degree. C.) 281.0 (138.3.degree. C.) 7.21 (865.2 g/liter) M--xylene -54.2 (-47.9.degree. C.) 282.4 (139.1.degree. C.) 7.23 (867.6 g/liter) O--xylene -13.3 (-25.2.degree. C.) 292.0 (144.4.degree. C.) 7.37 (884.4 g/liter) __________________________________________________________________________
Principal sources are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range of 10 to 32 wt. % ethylbenzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Paraxylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para- and orthoxylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes. At present, several xylene isomerization processes are available and in commercial use.
The isomerization process operates in conjunction with the product xylene separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
It will be apparent that separation techniques for recovery of one or more xylene isomers will not have material effect on the ethylbenzene introduced with charge to the recovery isomerization "loop". That compound, normally present in eight carbon atom aromatic fractions, will accumulate in the loop unless excluded from the charge or converted by some reaction in the loop to products which are separable from xylenes by means tolerable in the loop. Ethylbenzene can be separated from the xylenes of boiling point near that of ethylbenzene by extremely expensive "superfractionation". This capital and operating expense cannot be tolerated in the loop where the high recycle rate would require an extremely large distillation unit for the purpose. It is a usual adjunct of low pressure, low temperature isomerization as a charge preparation facility in which ethylbenzene is separated from the virgin C.sub.8 aromatic fraction before introduction to the loop.
Other isomerization processes operate at higher pressure and temperature, usually under hydrogen pressure in the presence of catalysts which convert ethylbenzene to products readily separated by relatively simple distillation in the loop, which distillation is needed in any event to separate by-products of xylene isomerization from the recycle stream. For example, the Octafining catalyst of platinum on silica-alumina composite exhibits the dual functions of hydrogenation/dehydrogenation and isomerization.
The rate of ethylbenzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethylbenzene approach to equilibrium. Temperature change within the range of Octafining conditions, i.e. 830.degree. F. (443.degree. C.) to 900.degree. F. (482.degree. C.), has but a very small effect on ethylbenzene approach to equilibrium.
Concurrent loss of ethylbenzene to other molecular weight products relates to % approach to equilibrium. Products formed from ethylbenzene include C.sub.6 +naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethylbenzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecular weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethylbenzene has been found responsible for a relatively rapid decline in catalyst activity and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible to relate catalyst stability (or loss in activity) to feed composition (ethylbenzene constant and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
The utilization of zeolites of the ZSM-5 class, (e.g. ZSM-5, ZSM-12, ZSM-35 and ZSM-38), for xylene isomerization is described in U.S. Pat. Nos. 3,856,871 and 3,856,873.
A significant improvement arose with the introduction of catalysts such as zeolite ZSM-5 combined with a Group VIII metal such as nickel or platinum as described in Morrison U.S. Pat. No. 3,856,872. It is disclosed in this Morrison patent that the catalyst be preferably incorporated in a porous matrix such as alumina. The Group VIII (hydrogenation) metal may then be added after incorporation with the zeolite in a matrix by such means as base exchange or impregnation. In the process of the U.S. Pat. No. 3,856,872 patent, ethylbenzene is converted by disproportionation over this catalyst to benzene and diethylbenzene. At temperatures in excess of 800.degree. F. and using a catalyst comprising a zeolite of the ZSM-5 class and of reduced activity, ethylbenzene and other single ring aromatics are converted by splitting off side chains of two or more carbon atoms as described in copending application Ser. No. 914,645, filed June 12, 1978 now U.S. Pat. No. 4,188,282. A particularly preferred form of zeolite ZSM-5 disclosed in said copending application is formed by the crystallization of the zeolite from a solution containing metal ions, such as platinum. This procedure shall hereinafter be referred to as "co-crystallization".
The use of zeolites characterized by a silica to alumina mole ratio of at least 12 and a Constraint Index in the approximate range of 1 to 12, i.e. the ZSM-5 class of zeolites, in conjunction with a Group VIII metal for aromatics processing, is disclosed in U.S. Pat. Nos. 4,101,595 and 4,101,597. Low pressure xylene isomerization using a zeolite catalyst such as ZSM-5 without a metal function is described in U.S. Pat. No. 4,101,596.
A further improvement in xylene isomerization, as described in U.S. Pat. No. 4,163,028, utilizes a combination of catalyst and operating conditions which decouples ethyl benzene conversion from xylene loss in a xylene isomerization reaction, thus permitting feed of C.sub.8 fractions which contain ethyl benzene without sacrifice of xylenes to conditions which will promote adequate conversion of ethyl benzene.
That improved process of the U.S. Pat. No. 4,163,028 patent utilizes a low acid catalyst, typified by zeolite ZSM-5 of low alumina content (SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of about 500 to 3000 or greater) and which may contain metals such as platinum or nickel. In using this less active catalyst, the temperature is raised to 800.degree. F. (427.degree. C.) or higher for xylene isomerization. At these temperatures, ethylbenzene reacts primarily via dealkylation to benzene and ethylene rather than via disproportionation to benzene and diethylbenzene and hence is strongly decoupled from the catalyst acid function. Since ethylbenzene conversion is less dependent on the acid function, a lower acidity catalyst can be used to perform the relatively easy xylene isomerization, and the amount of xylenes disproportionated is eliminated. The reduction of xylene losses is important because about 75% of the xylene stream is recycled in the loop, resulting in an ultimate xylene loss of 6-10 wt. % by previous processes. Since most of the ethylbenzene goes to benzene instead of benzene plus diethylbenzenes, the product value of the improved process is better than that of prior practices.